Research in our group is focused on the theory of ultracold atoms, in particular on few and many-body effects and - more recently - also on the back action between atoms and the light field in cavities. We are using largely analytical techniques from many-body physics and quantum field theory. An important aspect of our research is its direct connection with experiment, which is the basis for a close interaction with a number of experimental groups in this field, both within the Munich Quantum Center and internationally.
Cold atoms provide idealized and widely tunable model systems for the study of few and many-body phenomena, which have no counter part in condensed matter physics. Their interaction strength is adjustable either directly by Feshbach resonances or in optical lattices, where atoms are trapped in artificial crystals due to light forces. The figure shows the origin of the area-law dependence of the decay of parity order in two-dimensional Mott insulators (from F. Piazza, P.Strack and W. Zwerger, Ann Phys. 339, 135 (2013) [arXiv:1305.2928]).
Current topics of research are fermionic gases near the limit of infinite scattering length, in particular the connection between thermodynamic properties and short-distance correlations, which follow from the operator product expansion. Applications of these results appear in radio-frequency spectroscopy and in transport coefficients like the shear viscosity or the thermal conductivity.
The figure represents the shear viscosity to entropy ratio of the unitary Fermi gas in comparison with known asymptotes. The red diamond indicates the critical temperature for the superfluid transition.
(from T. Enss, R. Haussmann and W. Zwerger, Ann. Phys. 326, 770 (2011) [arXiv:1101.5594])
In few-body physics, we are studying the scaling laws in Efimov-type bound states of three atoms and their possible extension to a universal sequence of bound states up to an arbitrary number of particles. The figure represents the Universal spectrum of the bound state energies of Efimov trimers for open and more closed channel dominated Feshbach resonance (from R. Schmidt, S.P. Rath and W.Zwerger, EPJB 85, 386 (2012),[arXiv:1201.4310)
Regarding cold atoms in optical lattices, our current interest is focused on hidden order and topological edge states in Mott-insulating phases. In a related context, we are also studying the back action and strong coupling between cold atoms and the light field e.g. in optical cavities, where external driving can lead to the formation of crystalline ordering of the atoms which appears simultaneous with a super-radiant state of the light field.